upgrade/I.1.2.0/galera: ensure Galera VIP is alive after reboot

When rebooting a node where Galera & Keepalived (as a master) are running, we need to ensure that the MySQL VIP is running before starting to upgrade OpenStack databases.
Otherwise, it will lead to MySQL timeout.

Adding a new variable to group_vars/all: galera_vip (mandatory)

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>